1. Field of the Invention
The present invention relates to a process of forming a deposited film, a process of producing a semiconductor element substrate, and a process of producing a photovoltaic element. More particularly, the invention concerns a process of producing a semiconductor element such as a photovoltaic element or the like without curl deformation by incorporating a step of eliminating curl deformation.
2. Related Background Art
In recent years various research and development has been conducted toward practical use of solar power generation with solar cells (photovoltaic elements). In order to adapt the solar cells to the demand for power, there are such requirements that the solar cells used have sufficiently high photoelectric conversion efficiency and excellent reliability and that they can be manufactured in volume.
Amorphous silicon (hereinafter referred to as xe2x80x9ca-Sixe2x80x9d) solar cells are drawing attention, because they can be produced at a low cost and have high volume productivity, as compared with the solar cells made using crystalline Si or the like. The reason is that readily available gas such as silane or the like is used as a source gas and it is decomposed by a glow discharge whereby a deposited film such as a semiconductor film or the like can be formed on a relatively inexpensive belt-like substrate such as a metal sheet, a resin sheet, or the like. There are various suggestions as to processes and apparatus for producing the a-Si solar cells.
FIG. 1 is an example of a schematic sectional view of a single type a-Si solar cell. In FIG. 1, reference numeral 101 designates a substrate, 102 and 103 a back surface reflecting layer (102 denotes a metal layer and 103 a transparent oxide layer), 104 to 106 semiconductor layers (104 is an n-type semiconductor layer, 105 an i-type semiconductor layer, and 106 a p-type semiconductor layer), 107 a transparent conductor layer, and 108 a collector electrode. The part 101 to 107 will be called a solar cell slab (hereinafter referred to as xe2x80x9cslabxe2x80x9d) for conveniencexe2x80x94sake.
For producing the a-Si solar cell, an electroconductive belt-like substrate of stainless steel or the like rolled in a roll form is preferably used as the substrate 101; as to the back surface reflecting layer 102, 103, silver, aluminum, copper, and so on with high reflectance are preferably used for the metal layer 102 and transparent oxides with moderate resistance, including zinc oxide, tin oxide, and so on, are preferably used for the transparent oxide layer 103. These are deposited by a continuous sputter apparatus of the roll-to-roll system as also disclosed in Japanese Patent Application Laid-Open No. 6-184745, etc.
As a film forming production apparatus of semiconductor layers, U.S. Pat. No. 4,485,125 discloses a continuous plasma CVD apparatus of the roll-to-roll system. In the apparatus, the n-type semiconductor layer 104 and the p-type semiconductor layer 106 are formed by RF plasma CVD and the i-type semiconductor layer 105 by RF plasma CVD or by microwave (xcexcw) plasma CVD disclosed in Japanese Laid-open Patent Application No. 3-30419. Since the microwave has a high frequency, the energy density can be made higher than in the case of RF and it is thus suitable for efficient generation of a plasma at a low pressure and maintenance thereof. It is known that the microwave prevents polymerization of active species, which is the cause of degrading characteristics of a deposited film, to obtain the deposited film with high quality, suppresses evolution of powder of polysilane or the like in the plasma, and achieves drastic increase of the deposition rate.
The transparent conductor layer 107 is preferably formed by use of SnO2, In2O3, ITO (In2O3+SnO2) films, and so on having excellent characteristics including transparency to visible light and electric conductivity, and is formed by the continuous sputter apparatus of the roll-to-roll system.
The collector electrode 108 is formed, for example, in such a manner that after formation of the transparent conductor layer 107, a slab of a desired size is cut out of the roll and wires of copper, silver, or the like are attached thereto through a line of solar cell module steps.
For volume production of the solar cells by the roll-to-roll system, the belt-like substrate is lengthened and widened; for further reduction of cost, it is desired to decrease the thickness of the substrate.
However, the lengthening and widening of the substrate poses the problem that during formation of each deposited film, particularly, during formation of the back surface reflecting layer and semiconductor layers, the substrate is thermally deformed to curl because of the heat from a heater and the plasma. With decrease in the thickness of the substrate, there arises the problem that the substrate is subject to deposited membrane stress of each deposited film, to become easier to curl. Such problems arise more or less in forming a deposited film by the roll-to-roll system, without being limited to the solar cells.
The curl deformation of the substrate poses problems of occurrence of flaws, defects of the collector electrode, and so on and thus possibly degrades the yield and appearance of the solar cells considerably, in succeeding steps to formation of the transparent conductor layer, specifically, in steps for formation of the solar cell module, including the slab cutting step, the collector electrode forming step by attachment of wires, and so on. These problems may also arise similarly in production of the other elements than the solar cells; for example, in production of elements including semiconductor elements such as sensors or the like, liquid crystal elements (also including elements that cannot be mentioned as semiconductor elements), and so on.
An object of the present invention is, therefore, to form the semiconductor elements, particularly photovoltaic elements, with excellent appearance at a good yield, solving the problems as stated above.
According to one aspect of the present invention, there is provided a process of forming a deposited film on a belt-like substrate by a roll-to-roll system, the process comprising the step of eliminating a curl deformation of the belt-like substrate resulting from application of a deformation stress, by exerting an external stress on a non-deposition surface of the belt-like substrate.
According to another aspect of the present invention, there is provided a process of producing a semiconductor element substrate, comprising the step of depositing at least a semiconductor layer on a belt-like substrate by a roll-to-roll system, the process further comprising the step of eliminating a curl deformation of the belt-like substrate resulting from application of a deformation stress, by exerting an external stress on a non-deposition surface of the belt-like substrate.
According to yet another aspect of the present invention, there is provided a process of producing a photovoltaic element, comprising the step of depositing at least a semiconductor layer and a transparent conductor layer on a belt-like substrate by a roll-to-roll system, the process further comprising the step of eliminating a curl deformation of the belt-like substrate resulting from application of a deformation stress, by exerting an external stress on a non-deposition surface of the belt-like substrate.
The deformation stress includes thermal deformation stress in formation of a deposited film such as a semiconductor layer, a transparent conductor layer, or a back surface reflecting layer, and internal stress of the deposited film.
The belt-like substrate is preferably an electrically conductive substrate.
The external stress is preferably a deformation stress to effect plastic deformation of the belt-like substrate. The external stress is preferably exerted by use of a curl corrector, and the curl corrector is preferably a cylindrical roller type curl corrector. The external stress is preferably exerted after formation of the deposited film such as the transparent conductor layer, the semiconductor layer, or the back surface reflecting layer.
The present invention can solve the problems including the occurrence of flaws, the defects of the collector electrode, etc. in the succeeding steps including the slab cutting step, the step of forming the collector electrode by attachment of wires, and so on, by incorporating the step of eliminating the curl deformation generated in the belt-like substrate, by exerting the external stress to effect plastic deformation to the non-deposition surface.